This invention relates to an intake system for four-cycle engines and more particularly to an improved induction system that offers significantly better low and mid-range performance than prior art systems.
As is well known, engines and particularly those operating on the four-cycle principle normally have at least some overlap in their valve timing. That is, it is a common practice to effect opening of the intake valve while the piston is still undergoing the exhaust stroke. This is done to insure good charging efficiency at high speeds. During high speed running, there is a flow inertia in the intake charge that is sufficient to prevent any exhaust gases from being expelled from the chamber into the intake system. At lower speeds, however, there is some backflow into the intake passage which reduces charging efficiency. Although the problem may be obviated by employing valve timing with no overlap, high speed performance and maximum power output are severely restricted if this is done.
It has been proposed to avoid the backflow of exhaust gases into the intake system, even though a large valve overlap may be employed, by providing a check valve in the intake passage slightly upstream of the intake port. The use of a check valve in this location, normally a reed-type valve, will improve low speed and mid-range running. However, the use of a such a check valve provides a resistance in the flow passage that can reduce maximum power output.
It is, therefore, a principal object of this invention to provide an intake system for an engine that improves low speed torque and power output withut substantially affecting maximum power output.
It is another object of this invention to prove an induction system for an engine that provides improved performance at the low and mid-ranges without significantly restricting maximum output.